2020-12-3: make check and linter errors are unclear

[baileythegreen]

The errors produced by the make check and the yamllint CI checks are unclear to anyone who either did not write those files, or who has not already devoted some time into understanding what those files do.

I propose:

• Adding informative errors to their output. This is easy to do in Python, so I think could be incorporated into the utils/check-glossary.py script fairly easily. This would at least help with some of this.
• Adding some information about the CI to the contributing documents, explaining the types of things that are checked, summarising the errors that are likely to arise, and what might cause them.

Both of these things will make it easier for more people to contribute to this project confidently, without needing to be comfortable debugging multiple kinds of programs to get their PRs to pass the checks.